A traditional electrostatic loudspeaker comprises a conductive membrane disposed between two perforated conductive backplates to form a capacitor. A DC bias is applied to the membrane and an AC signal voltage is applied to the two backplates. Voltages of hundreds or even thousands of volts may be required. The signals cause an electrostatic force to be exerted on the charged membrane, which moves to drive the air on either side of it.
In U.S. Pat. No. 7,095,864, there is disclosed an electrostatic loudspeaker comprising a multilayer panel. An electrically insulating layer is sandwiched between two electrically conducting outer layers. The insulating layer has circular pits on one of its sides. It is said that when a DC bias is applied across the two conducting layers, portions of one of the layers are drawn onto the insulating layer to form small drumskins across the pits. When an AC signal is applied, the drumskins resonate, and parts of that conducting layer vibrate to produce the required sound.
In WO 2007/077438 there is disclosed an further type of electrostatic loudspeaker comprising a multilayer panel. An electrically insulating layer is sandwiched between two electrically conducting outer layers. In this arrangement, one of the outer conducting layers is perforated and, for example, may be a woven wire mesh providing apertures with a size of typically 0.11 mm.
In US 2009/0304212 there is disclosed an electrostatic loudspeaker comprising a conductive backplate provided with an array of vent holes and an array of spacers. Over this is positioned a membrane comprising a dielectric and a conductive film. The space between the backplate and the membrane is about 0.1 mm and it is said that a low voltage supplied to the conductive backplate and the conductive film will push the membrane to produce audio.
One problem with electrostatic loudspeakers of this type is obtaining sufficient displacement of the membrane. WO 2012/156753 discloses an electrostatic transducer comprising an electrically conductive first layer having through apertures, a flexible insulating second layer over the first layer, and a flexible electrically conductive third layer disposed over the second layer. Spaces are provided between the first and second layers or between the second and third layers. Spaces between the first and second layers allows greater freedom of movement of the second and third layers, allowing greater displacement of the second and third layers. Spaces between the second and third layers were also found to improve acoustic performance.
However, there remains a need for further improvement in the acoustic performance of electrostatic transducers of this type.